Falklands War.
Against all odds, this was a war that no one thought Britain had the will to win but our professionalism and perseverance on the far side of the world showed that Britain's decline in the 1970's was to end and be reversed.
It was also likely the last "proper war" of the 20th century. It was not a proxy war or just a pawn in the machinations of the Cold War but a conflict harking back to the conflicts of the 1930's: a military Junta invading a small defenceless country in nothing but a fascistic flexing of military might.
It was a war where the morality was clear, the mission was clear, while simultaneously the task seemed impossible and pushed everyone to their limits only not only was victory achieved but was achieved with amazing success. And even when disaster struck, such as the Exocet missiles, the conviction of the armed forces, the politicians and the people did not falter.
Least favourite war would have to be the current in Afghanistan, not because I disagree with the principal of us being there but that equipment shortages and terrible management at a political level are dooming such a righteous cause. But with any luck (perhaps a new government) the failures can be reversed.